I have a very weak / almost nonexistent sense of smell. what could it mean?

Nasal obstruction: The most common cause of diminished smell is due to obstruction of the nasal passages resulting in the inability of molecules of air to reach the olfactory (smell) nerve) in the roof of the nasal cavity. Obstruction may be due to nasal allergy, mechanical obstruction, nasal polyps, certain drug abuse. Sometimes these causes can be treated.